\documentclass[a4paper]{scrartcl}
\usepackage{amssymb, amsmath} % needed for math
\usepackage[utf8]{inputenc} % this is needed for umlauts
\usepackage[ngerman]{babel} % this is needed for umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
\usepackage[margin=2.5cm]{geometry} %layout
\usepackage{hyperref}       % links im text
\usepackage{csquotes}       % nice quotes
\usepackage{parskip}        % I don't want indentation
\usepackage{color}
\usepackage{framed}
\usepackage{enumerate}  % for advanced numbering of lists
\usepackage[locale=DE,per=slash]{siunitx}    % for propper units
\DeclareSIUnit{\EUR}{\text{~Euro}}
\usepackage{multicol}
\usepackage{titlesec}
\titleformat{\section}{\normalfont\Large\bfseries}{\S\thesection}{1em}{}
\clubpenalty  = 10000   % Schusterjungen verhindern
\widowpenalty = 10000   % Hurenkinder verhindern

\hypersetup{
  pdfauthor   = {Martin Thoma},
  pdfkeywords = {Untermietvertrag},
  pdftitle    = {Untermietvertrag}
}

% Anpassen %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\newcommand{\HauptmieterVorname}{Martin}                            %
\newcommand{\HauptmieterNachname}{Thoma}                            %
\newcommand{\HauptmieterAnschrift}{Parkstraße 17, 76131 Karlsruhe}  %
\newcommand{\UntermieterVorname}{Max}                               %
\newcommand{\UntermieterNachname}{Müller}                           %
\newcommand{\UntermieterAnschrift}{}                                %
\newcommand{\UntermieteBeginn}{01.06.2014}                          %
\newcommand{\UntermieteEnde}{31.08.2014}                            %
\newcommand{\Monatsmiete}{200}                                      %
\newcommand{\Kaution}{250}                                          %
% Mietgegenstand
\newcommand{\Strasse}{Parkstraße} % Deine Straße                    %
\newcommand{\Hausnummer}{17}      % Deine Hausnummer                %
\newcommand{\PLZ}{76131}          % Deine PLZ                       %
\newcommand{\Ort}{Karlsruhe}      % Dein Ort                        %
\newcommand{\Wohnflaeche}{19}     % in m^2                          %
% Konto
\newcommand{\Kontoinhaber}{Martin Thoma}                            %
\newcommand{\Kontonummer}{}                                         %
\newcommand{\IBAN}{TODO}                                            %
\newcommand{\Bank}{TODO}                                            %
\newcommand{\BLZ}{TODO}                                             %
\newcommand{\Verwendungszweck}{Miete von [Name des Untermieters]}   %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Begin document                                                    %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{document}
I, \HauptmieterVorname~\HauptmieterNachname, hereby confirm that I have received \line(1,0){75}
Euro from \UntermieterVorname~\UntermieterNachname.

I, \UntermieterVorname~\UntermieterNachname, hereby confirm that I have received 4 keys (house key,
floor key, flat key and room key) from \HauptmieterVorname~\HauptmieterNachname.

\vspace{1.5cm}
\line(1,0){250}\\
\vspace{-0.3cm}
{\scriptsize Date, Signature of \HauptmieterVorname~\HauptmieterNachname}\\


\vspace{1cm}
\line(1,0){250}\\
\vspace{-0.3cm}
{\scriptsize Date, Signature of \UntermieterVorname~\UntermieterNachname}\\
\end{document}